The barrier function of gastrointestinal epithelium confers high permselectivity to the GI tract. Several studies have demonstrated increased intestinal permeability to macromolecules that may be linked to the pathogenesis of Crohn's Disease. The determinants of gastrointestinal permeability are complex and incompletely understood. We have established the presence of an unusual barrier to the permeation of ammonia (a weak base) at the apical borders of rabbit colonic-crypt cells. Under Clinical Investigator Award K08 DKO2410, we recently determined that a permeability barrier exists as well to the weak acids CO2 (a gas, like NH3) and butyric acid, all of which are abundant products of fermentation in the colonic lumen. Each of these molecules possesses neutral lipophilic forms that permeate most cell membranes, affecting intracellular pH. Indeed, using a recently- developed preparation, we found that polarized colonic surface-cells, unlike their progenitors located in the crypt, possess high apical permeability to NH3, butyric acid, and CO2. The barrier to gases/lipophilic molecules in colonic glands is likely important to the overall barrier function of gastrointestinal epithelium. As such, it is important to know how altered apical "micromolecular" barriers affect epithelial cell health, and whether an altered micromolecular barrier precedes and/or predisposes to the increased "macromolecular" permeability associated with Crohn's Disease: As the unusual barrier function of colonic crypts has only recently been appreciated, no data as yet exist on its relationship to human disease. Present animal models of inflammatory bowel disease do not represent optimally the pathophysiologic processes that culminate in human Crohn's disease. Fortunately, we can study crypt cell intracellular pH in perfused human crypts. In recent work, we observed in human crypts an apical barrier to NH3 diffusion but not to NH4+ transport in all patients. In addition, we have been able to kinetically characterize acid-base transport among cells that comprise human crypts. These technical advances, applied to crypts obtained by sigmoidoscopic biopsy, permits direct study of micromolecular barriers in humans, and enables evaluation of the role of altered micromolecular barriers in the pathogenesis of Crohn's Disease. Although studies of the fundamental biology of apical acid-base barrier phenomenon have broad application to membrane biology, studies outlined in this application are focused long-term on the biology of the barrier and colonocyte pH as it relates to Crohn's Disease.